This invention relates to monitoring and control of electrical transmission lines, and to a field that is generally known as transmission line protective relaying.
AC power transmission lines are often protected by so-called protective relaying systems which operate upon occurrence of a fault to trip circuit breakers that protect the transmission line from damage and isolate the faulted portion of transmission line from the rest of an overall transmission system. Typically, the section of transmission line to be protected extends between terminals called local and remote terminals, and substantially identical protective subsystems are located at the remote and the local terminals. Current on the transmission line (or on individual phases thereof) is sensed at both the local and the remote terminals, and information concerning the current is transmitted over a communications channel from the remote terminal to the local terminal, and vice versa. Each subsystem includes means for comparing the local and remote current readings and for generating trip control signals as a function of the comparison. The trip control signals operate, under certain conditions, to trip circuit breakers at the respective locations when the subsystems detect a condition that indicates an internal fault; i.e., a fault within the protected section of transmission line. The subsystems are designed to avoid generation of tripping signals when detecting conditions that indicate a fault external to the protected section of transmission line. The communications link may be a telephone line or the like, or a dedicated line for carrying information between the local and remote terminals.
In order to ensure that there is a fast protective response to a fault in the transmission line being monitored, information is sampled and sent at a fairly high rate. This raises the bandwidth requirements for the communications channel between the local and and remote terminals. Also, in order to correlate information received from a remote terminal with the corresponding measurement taken at a local terminal, it is necessary to estimate the delays encountered in generating and communicating the remote information to the local terminal. This can be problematic if the processing technique is not forgiving of errors in the estimated delays, since these delays can vary substantially.
It is among the objects of the present invention to reduce the amount and rate of information that needs to be generated and transmitted over a communications channel used for power line monitoring and control, while maintaining the integrity of the monitoring and control functions. It is among the further objects of the present invention to reduce dependence on the accuracy of estimation of communications channel delays. It is also among the objects hereof to reduce the complexity and inefficiency of existing protective relaying techniques.